Temperature compensation



o. E. Dow Y 2,109,880

TEMPERATURE COMPENSATION Filed oct. so, 1935 2 sheets-sheet 1 March 1, 1938.

March 1, 1938. O. E. DOW

TEMPERATURE COMPENSATION `2 Sheets-Sheet 2 Filed Oct. 30, 1955 mE. /w

WHW Y Patented Mar. 1, 1938 PATENT OFFICE 2,109,880 TEMPERATURE COMPENSATION vOrville E. Dow, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Appli-cation october so, 1935, serial No. 47,379

18 Claims.

This invention relates to temperature compensation devices, and particularly to a condenser with an adjustable temperature coefficient for use in low power factor resonator circuits.

It is well known that the value of the capacitance of a condenser is dependent, among other things, upon the spacing between the plates of the condenser, and the area of the plates. This spacing, and also the area of the plates, is subject to change in size with change in temperature, for which reason there is frequently caused an undesired change in the electrical constants of a high frequency circuit in which the condenser is used. A particular circuit in which such a change in capacitance of the condenser has been found to be especially harmful and undesired, is the low power factor line frequency control oscillator, and it is with respect to such a circuit that the invention will be described, although it is to 20 be understood that the invention is not limited thereto but may be used wherever high frequency selective'circuits, such as filters etc., are employed.

In the type of low power factor circuits in 25 which we are particularly concerned, there is employed, for controlling the frequency of an ultra high frequency circuit, a concentric line resonator having uniformly distributed capacitance and inductance in the form of inner and outer cylindrical conductors. Since the resonant frequency of such a line is dependent upon the overall length of the inner conductor as projected upon the outer conductor, it has Vbeen customary to provide means for maintaining the o overall length of the inner conductor of the line constant despite temperature fluctuations. Arrangements of this sort employing a rod of low temperature coeflcient with and without a bellows mechanism for controlling the overall length of the inner conductor, are described in copending Fred H. Kroger applications Serial No, 1,489, filed January l2, 1935, and Serial No. 5,058 (now United States Patent No. 2,077,800, grantedApril 20, 1937), filed February 5, 1935. Where special arrangements are not used in low power factor concentric line circuits for maintaining the overall length of the inner conductor constant, it is necessary to compensate for the variations in length of the line conductors with variations in temperature. The present invention finds use in both of the foregoing cases which employ concentric line resonators.

One of the 'objects of the present invention is to provide, in a low power factor circuit having a capacitor in series with a line resonator, means for maintaining theresonant frequency of the resonator constant despite temperature fluctuations.

Another object of the invention is to provide in such a circuit means for compensating for variations in the capacitance of the capacitor, which are due to variations in temperature. A further object is to provide such means from easily available construction materials.

Other objects and features will appear from a reading of the following description, which is accompanied by drawings wherein:

Fig. 1 illustrates the invention as applied to a transmitter circuit employing a capacitor in series with a low power factor concentric line; and

Figs. 2 and 3 illustrate other embodiments of the invention as applied to low power factor concentric line circuits.

Referring to Fig. '1, there is shown an oscillation generator circuit for a transmitter, provided with a low power factor concentric line resonator coupled to two electron discharge device oscillators O and O. Although this particular transmitter circuit forms no part of the present invention per se, and is completely described in a copending application, Serial'No. 38,707, filed August 31, 1935, by Fred H. Kroger, to which reference is made for a more detailed description, the pertinent portion of the transmitter circuit which cooperates with the compensation device of the present invention will be described.

The line resonator controls the frequency of the oscillators O and O and consists of an inner conductor I and an outer conductor 2 having substantially uniformly -distributed inductance and capacitance. Line I, 2 is maintained constant in length by means of an invar rod I and bellows Il which are connected to the free end of the inner conductor I, as shown, and in a manner which is more adequately described in copending application, Serial No. 1,489, supra. Oscillators O and O have their grids connected to the control line by means of very short grid leads GL and GL' which connect with the metallic plate 4 of the capacitor 4, 5, the plate 5 comprising the end plate of the outer conductor 2.

Line I has an electrical length which is approximately equal to one-quarter of the length of the operating wave, as measured from its free end to the point X to which the inductance 3 is connected. That portion of the line which extends beyond X and up to plate 4 is used to tune out the capacitance of capacitor 4, 5 which is placed in series with the line. Capacitor 4, 5 is made adjustable to provide a convenient method of setting the line circuit exactly at frequency. For providing stability of operation, the control grids of the oscillators O and O are arranged to be capacitive to ground for all frequencies except the one desired, and this is accomplished by connecting these grids through very short leads, to the condenser plate 4, thus avoiding any possibility of parasitic oscillations. Since the frequency control line i, 2 has no radiation resistance and very little electrical resistance, it constitutes an electrical circuit of high selectivity, and it is this quality which is utilized to stabilize the frequency of the master oscillator devices O and O'. Oscillators O and O are arranged in parallel relation in order to enable the transmitter to continue to function in case one device should fail in any manner which does not overload the power supply.

Each oscillator O and Ol is arranged to have its individual anode 'circuit constituting the anode tuning inductor I2 and by-pass condenser 9. By adjusting this inductance I2, the load between the two oscillator tubes is equally divided. A source of supply -l-B maintains the anodes of the oscillators at a suitable positive potential. In.- ductor 3 is merely a choke to provide a direct current return path from the grids of the oscillators to the respective cathodes. The connection from inductor 3 could, theoretically, be placed anywhere along the inner conductor l, provided the proper choke is supplied. Practically, the most ecient and convenient point of connection between the conductor l and inductor 3 is Where the inner conductor has approximately Zero radio frequency potential.

The value of capacitor 4, 5 determines what amount of power shall be dissipated in the line for a given amount of grid excitation of the oscillators O and O'. This capacitor is in series with the line l and is provided with temperature compensation means which, in conjunction with the invar rod Ill and bellows Il, maintains constant all factors which inuence the frequency of the oscillators. It will thus be appreciated that to maintain the resonant frequency of the oscillator circuit constant with variations in temperature, the capacitance of the capacitor 4, 5 must remain constant, provided, of course, that the inner conductor l is temperature compensated by such means as shown. To effect this result the temperature coeiiicient of linear expansion of the condenser spacers is designed to be twice the temperature coeficient of expansion of the material in the condenser plates 4, 5. The foregoing relation of temperature coeflicients will be apparent if we assume that the capacitor takes the form of two discs, as is the case here involved, each disc having the radius of r centimeters and spaced do centimeters apart. Assuming that the condenser plates 4, 5 are made of a material which has a linear temperature coefficient of expansion of A per degree centigrade, the capacity of the condenser at a fixed temperature t1 will be:

- (If T is less than t1, it must be given a negative sign.) Inasmuch as the quantity A is very small, A2T2 can be neglected and the capacitance then becomes:

C Kr2(1+2AT) It is thus evident that for the capacitance C to be independent of temperature, A2 must be equal to 2A; in other words, the linear temperature coefficient of expansion of the condenser spacers must be twice the linear temperature coefiicient of expansion of the condenser plates.

In practice, the condenser plates 4, 5 are made of copper, for which reason substances for use as spacers will be required which have an expansion coefficient of .33 10-4 per degree centigrade. A material with such a temperature coefficient and suitable mechanical and electrical properties is exceedingly difficult to find, but by means of the present invention the required characteristic is obtained with readily available and common construction materials usually at hand.

Fig. l is shown provided with means for making the capacitance of the line capacitor 4, 5 independent of temperature, or ,for giving the capacitor either a positive or negative characteristic to compensate for any part of the circuit which would tend to change the frequency of the oscillators O, O with variations in temperature. Due to the size of the plates 4, 5 there have been shown in this figure two identical compensation devices, although it will be appreciated that if desired one or any number may be used for effecting the desired compensation. By providing a supporting rod 5, preferably made of copper, and which is preferably longer than other supporting rods 'l made of aluminum, and by varying the position of the yoke 8, the capacitance 4, 5 can be given a positive, Zero, or any negative temperature coelicient. Since copper has a linear temperature coefficient of expansion of 16.8 1OG per degree centigrade, and aluminum has a temperature coefficient within the range from 23.1 to 25.5 106 per degree centigrade, it will be appreciated that the aluminum rods 1 cause the condenser spacing between plates 4 and 5 to decrease as the temperature is increased, while the copper rod 6 causes the spacing between the plates 4, 5 to increase with increase in temperature. Consequently, by means of a suitable choice of dimensions and materials, the dielectric spacing of the plates 4, 5 can be made to increase at a rate which will just counterbalance the effect of the increase in area of the plates 4, 5, or the increase in spacing between the plates 4, 5 may be exaggerated by using for rod 6 a material of a greater temperature coefficient than the temperature coefficient for the material of rods 1, or the converse may be provided. If it is desired to give the capacitor 4, 5 a zero temperature coefficient, and the rods are of materials set forth above, then yoke 8 is so located that the spacing d, between the plates 4, 5, has the following characteristic:

where do is the spacing at the operating temperature and T is the variation in degrees centigrade from that temperature.

The foregoing method of obtaining temperature compensation is extremely accurate only when the component parts of the unit are kept at the same temperature, and for this reason it is preferred that the component parts be made rather large and of good heat conducting material. In some cases, if desired, the parts of the device may be kept at the same temperature by blowing air up through the line I, 2 and past the temperature compensation devices 6, 1, 8.

Fig. 2 is another embodiment of the invention and shows connected to the movable plate 4, which is in circuit with the inner conductor of the line, not shown in thisfigure for the sake of simplicity, a supporting rod I4 insulated from the plate by means of insulator I5. Connected to the upper plate 5, which is in contact with the outer conductor of the line, is an internally threaded cylinder I3 which is designed to have av different temperature coefficient of expansion than the rod support I4. This rod is connected to the cylinder I3 by means of an internally and externally threaded ring I9 and element I6 which has an external thread of different pitch than its internal thread. The internally and externally threaded ring I9 is screwed up or down to obtain the desired temperature coefficient for the condenser while part I6 is employed to obtain a micrometer adjustment of the condenser. Elements I'I and I8 lock the unit into position after final;` adjustments are: made. Element I'I is threaded so as to `engage the thread on rod I4, but is not threaded on its bottom portion which presses against element I8 when it (I'I) is screwed down on rod I4. Element II does not, however, engage ring I9.

Elements I4, It,v I1, I8 and I9 may all be made of the same material, such as brass which has a temperature coefficient cf expansion of while elements 4, 5 and I3 may be made of copper, which has a temperature coefficient of Using these materials for the elements as an illustration, and taking the reference plane as the lower surface of plate 4, it will be evident that with an increase of T degrees in temperature the upper surface of plate 4 will move up away from the reference plate by an amount where t is thethickness of plate 4. The lower surface of upper plate 5 will move up by an amount 18.6 106TZ-16.8 10-6TY, where Z is the vertical distance from the reference plane to the lower surface of brass link I9 and Y is the vertical distance from the lower surface of plate 5 to the lower 'surface of element I9. The foregoing expression is correct because the expansion of brass supporting elements I4 and I6 is partly cancelled by the expansion of copper elements I3 and 5, considering, of course, only dimensions perpendicular to the plates 4 and 5. Consequently the change in the spacing between plates 4 and 5 is Al=18.6 10-6TZ-16.8 106(t-IY) T (a) Since it has been shown previously that the spacing must vary in the following manner to keep the capacitance constant with a change of T degrees in temperature d=az 1+2` 1613x104@,

the change in spacing must be The values for Z and Y may, of course', 'be obtained from the foregoing Equations (a) (b) and (c). If, for example do=-t:.25 inch, then Y will be 1.83 inches and Z will be 2.33 inches.

Element I6 is employed to adjust the spacing do, and connecting link I9 determines the length ofY Z and Y. If connecting link I9 is threaded inside and outside with the same pitch thread of, let us say, sixteen threads per inch, and the outside thread of link I6 is the same as I9 but its inside thread which engages with rod I4 has either a larger or a smaller pitch, let us say for this particular case twenty threads per inch, and if link I9 is held stationary with respect to cylinder I3, and element I6 is turned one revolution, the spacing do will then be changed by an amount equal to g-/go inch.

In the illust-ration given above, if insulator I were of the form of that shown in Fig. 1, then it would be necessary for cylinder I3 to have a greater temperature coefhcient of expansion than element I4, in which case I3 may be made of brass while elements I4, I6 and I9 may be made of copper. It does not matter what the insulator is made of as long as its electrical qualities are satisfactory. It is the shape of the insulator rather than the material from which it is made which determines whether or not cylinder I3 should have a greater temperature coefcient of expansion than element I4.

Fig. 3 shows a further modification of the invention employing a capacitor in series with the concentric line with temperature compensation means. In this figure, if it is assumed that the inner conductor has no means for compensating for variations in temperature, the capacity of the series condenser composed of plates 4 and 5 must be made to decrease with an increase in temperature in order to maintain the resonant frequency of the plate circuit constant. 'This follows from the fact that the impedance due to the concentric conductor line may be represented by the following equations:

where Zo is the characteristic impedance of the line resonator, l is the length of the inner conductor of the line resonator, and is the operating wave length. Since the reactance of the series capacitor C at the operating frequency must be Very nearly equal to but opposite in sign to the reactance Z of the line, obeying the following equation:

where f is the operating frequency, is will be evident that if the inner conductor of the line is not temperature compensated, L will increase with an increase in temperature, thus increasing the reactance of the line. In order for this circuit to remain resonant at the same frequency, the reactance of the series capacitor must also increase by the same amount, and this indicates that the condenser spacing between plates 4 and 5 must increase to compensate both for the increase in area of the condenser plates and for the increase in length of the inside conductor I of the line I, 2. In the present instance, the desired degree of compensation is obtained by adjusting the lengths of posts 20 by means of clamps 2|. Posts 20 are made from a material, such as copper clad steel, with a lower temperature coefficient of expansion than the material used in the outer conductor 2, and are fastened to the plate 4` by means of insulators 22, which may be made of mycalex or any other good radio frequency insulator material. Post 20 is copper clad to reduce losses. `If the inner conductor I is provided with means for maintaining the overall length constant despite fluctuations of temperature, any one of the arrangements shown in Figs. 1, 2 and 3 may be employed to maintain the series'capacity of plate 4 and 5 independent of changes in temperature. In other words, the systems of all three figures may be used irrespective of whether the inner conductor I is temperature compensated by an invar rod and bellows arrangement, because the spacing of the series condenser in either event must increase with increase of temperature to compensate for the increase of area of the condenser plates. However, if the inner conductor of Fig. 3 increases with increase in temperature, the condenser spacing will increase at a greater rate with increase in temperature than it would if the length of the inner conductor were independent of temperature. In either case, the arrangement of the compensating device, in accordance with the invention will be the same, although the adjustment will be different.

vWhat is claimed is:

l. In combination, a low loss resonant circuit comprising coaxial inner and outer conductors coupled together at one of their adjacent ends by a capacitor, said capacitor comprising two spaced plates, one of said plates being directly fastened to one of said conductors, the other of said plates being fastened to said other conductor, and a plurality of metallic elements having different temperature coefficients of expansion connected between said plates for varying the spacing between said plates in a predetermined fashion in response to temperature fluctuations, whereby the capacitance of said capacitor is maintained substantially independent of changes in temperature.

2. In combination, a low loss resonant line circuit comprising coaxial inner and outer conductors coupled together by a capacitor, said capacitor comprising a plate fastened to one of said conductors and a plate fastened to the other of said conductors, and means connected between said plates for supporting said plates in spaced relation to each other, said means having different temperature coefficients of expansion for varying the spacing between said plates in predetermined fashion in response to fluctuations in temperature.

3. In combination, a low loss resonant line comprising inner and outer conductors, a condenser comprising a first plate supported by one of said conductors and a second plate fastened to said other conductor and spaced away from said first plate, metallic means mechanically linked to said first plate for supporting said second plate, said means having a different temperature coefficient of expansion than said one conductor supporting said first plate, whereby the effect on the resonant frequency of a change in length of said conductors due to a change in temperature is com.- pensated for by a Variation in the capacitance of said condenser.

4. In combination, a low loss resonant line having coaxial inner and outer conductors coupled together at one of their adjacent ends by a capacitor having first and second plates spaced away from each other, said first plate being mounted on said outer conductor, said second plate being fastened to said inner conductor and being supported by an element mounted on'but having a different temperature coefficient of expansion than said outer conductor.

5. In combination, a capacitor having a first plate and a second plate spaced away from said first plate, an insulator affixed to said second plate, a metallic supporting rod secured to said insulator and extending through apertures in said first and second plates, and a metallic element secured to said first plate and extending in the same direction as said rod, said rod and element having different temperature coefficients of expansion, and a link connecting said rod. and element together, whereby the spacing between said plates changes with change in temperature in dependence upon the difference in temperature coefficients of said rod and element.

6. Apparatus in accordance with claim. 5, characterized in this that said rod is made of brass and said element of copper.

7. Apparatus in accordance with claim 5, characterized in this that said rod is made of copper and said element of aluminum.

8. In combination, a capacitor having a first plate and a second plate spaced away from said first plate, an insulator afiixed to said second plate, a metallic supporting rod secured to said insulator and extending through apertures in said second and first plates, and a plurality of other rods secured to said first plate and extending in the same general direction as said first rod, said plurality of other rods being made of a material having a temperature coeflicient of expansion greater or less than said first rod, and a yoke connecting all of said rods together, whereby the spacing between said plates changes with change in temperature in dependence upon the difference in temperature coefficients of said rods.

9. In combination, a capacitor having a first plate and a second plate spaced away from said first plate, an insulator affixed to said second plate, a metallic supporting rod secured to said insulator and extending through apertures in said second and first plates, a metallic internally threaded tube secured to said first plate and surrounding said rod for at least a portion of its length, and an internally and externally threaded ring linking said tube at its inner surface with said rod, whereby movement of said ring adjusts the spacing of said plates, said tube and rod having different temperature coefficients of expansion, whereby the spacing between said plates changes with change in temperature in dependence upon the temperature coefficients of said rod and tube. e r

10. Apparatus in accordance with claim 9, including means for locking said ring in position.

11. Apparatus in accordance with claim 9, characterized in this that said tube and said plates the made of copper, and said rod and said ring are made of brass.

12. In combination, a capacitor having a first plate and a second plate spaced away from said first plate, a metallic cylinder supporting said first plate, said second plate being located within said cylinder, and a metallic support for said second plate, said last support also being located within said cylinder and mounted on said cylinder, said cylinder having a different temperature coefficient of expansion than said support for said second plate, whereby the spacing between said plates changes with change in temperature in predetermined fashion in dependence upon the temperature coefficients of said cylinder and support.

13. In combination, a low loss resonant circuit comprising two spaced parallel conductors slightly longer than one quarter Wave length, a condenser comprising a. rst plate supported by one of said conductors at one end, and a second plate supporting said other conductor at the adjacent end, and spaced away from said first plate, metallic means supporting said second plate, said means having a different temperature coefiicient of expansion than said one conductor supporting said rst plate, whereby the effect on the resonant frequency of a change in length of said conductors due to a change in temperature is compensated for by a variation in the capacitance of said condenser.

14. In combination, a low loss resonant circuit comprising two spaced parallel conductors slightly longer than one-quarter wavelength, a condenser comprising a first plate supported by one of said conductors at one end, and a second plate sup-porting said other conductor at the adjacent end, and spaced away from said first plate, metallic means supporting said second plate, said means having la different temperature coeicient of expansion than said one conductor supporting said rst plate, whereby the effect on the resonant frequency of a change in dimensions of said plates due to a change in temperature is compensated for by a Variation in the capacitance of said condenser.

15. In combination, a. low loss resonant line circuit comprising coaxial inner and outer conductors coupled together at one of their adjacent ends by a capacitor, said capacitor com.- prising a plate fastened to one of said conductors and a plate fastened to the other of said conductors, and a plurality of means symmetrically positioned around the axis of said line circuit for Vsupporting said plates in spaced relation to each other, each of said means comprising a plurality of rods having different temperature coefficients of expansion for varying the spacing between said plates in predetermined fashion in response to fluctuations in temperature.

16. In combination, a low loss resonant line circuit comprising coaxial inner and outer conductors coupled together at one of their adjacent ends by a capacitor, said capacitor comprising a plate fastened to one of said conductors and a plate fastened to the other of said conductors, and a plurality of means symmetrically positioned around the axis of said line circuit for supporting said plates in spaced relation to each other, said means comprising metallic elements having predetermined temperature coefficients of expansion for varying the spacing between said plates in predetermined fashion in response to fluctuations in temperature.

1'7. In combination, a low loss resonant line circuit comprising coaxial inner .and outer conductors coupled together at one of their adjacent ends by a capacitor, said capacitor comprising a plate fastened to one of said conductors and a plate fastened to the other of said conductors, and a plurality of means symmetrically positioned around the axis of said line circuit and connected between said plates for supporting said plates in spaced relation to each other, each of said means comprising a plurality of rods having different temperature coefficients of expansion for varying the spacing between said plates in predetermined fashion in response to fluctuations in temperature.

18. In combination, a capacitor comprising a pair of parallel plates symmetrically positioned with respect to a common axis passing transversely through the centers of said plates, a conductor in the form of a surface of revolution supporting one of said plates, and a plurality of means symmetrically positioned around said axis and connected between said plates for supporting said plates in spaced relation to each other, said means each including an element Whose temperature coeicient of expansion is different from the temperature coeflicient of expansion of said surface of revolution.

ORVILLE E. DOW. 

